Magnetic random access memory (MRAM) is a new technology that will likely provide superior performance over existing flash memory technology and may even replace hard disk drives in certain applications requiring a compact non-volatile memory device. In MRAM bits of data are represented by a magnetic configuration of a small volume of ferromagnetic material and its magnetic state that can be measured by magnetoresistive (MR) effect during a read-back operation. The MRAM typically includes an array of memory cells with each cell containing one magnetic tunnel junction (MTJ) element that can store at least one bit of data.
Conventional MRAM designs employ an array of MTJ elements that are based on a giant magnetoresistance (GMR) where the elements represent patterned thin film multilayers that include at least one pinned magnetic layer and one free magnetic layer separated from each other by a thin tunnel barrier layer. The free layer has two stable directions of magnetization that are parallel or anti-parallel to the fixed direction of magnetization in the pinned layer. Resistance of the MTJ depends on the mutual orientation of the magnetizations in the free and pinned layers and can be effectively measured. A resistance difference between the parallel and anti-parallel states of the MTJ can exceed 600% at room temperature.
The direction of the magnetization in the free layer can be changed from parallel to anti-parallel or vice-versa by applying an appropriate cross-point addressing magnetic fields to the selected MTJ, by passing a spin-polarized current through the selected junction, or by using a combination of the external magnetic field along with spin-polarized current simultaneously affecting the selected MTJ.
Majority of the current MRAM designs uses the free and pinned layers made of magnetic materials with in-plane direction of the magnetization (anisotropy) in the MTJ. The in-plane MRAM (i-MRAM) suffers from a large cell size, low thermal stability, poor scalability, necessity to use MTJ with a special elliptical shape, and from other issues, which substantially limit the i-MRAM capacity and its application at technology nodes below than 90 nm. MRAM with a perpendicular direction of magnetization (p-MRAM) in the free and pinned layers of MTJ can solve the above problems since the magnetic materials with the perpendicular magnetization have a high intrinsic crystalline anisotropy that provides MTJ with the high thermal stability, excellent scalability and a possibility using junctions having any shape. However, current designs of the p-MRAM suffer from high write current or from the necessity to use additional write conductors that limit MRAM density.
FIG. 1 shows a schematic view of a p-MRAM cell 10 according to the U.S. Pat. No. 6,845,038 (Shukh). The cell 10 employs a field induced switching mechanism of MTJ based on simultaneous effect of two intersecting magnetic fields. The MTJ comprises a pinned layer 14 and a free layer 18 both having their magnetizations directed substantially perpendicular to layers plane in their equilibrium states and separated from each other by a thin tunnel barrier layer 16. The p-MRAM cell 10 further comprises two intersecting write lines: a write word line WWL and a write bit line WBL; and two contact layers 12 and 22. Write currents IW1 and IW2 running through the lines WBL and WWL generate magnetic write fields HW1 and HW2, respectively. The orientation of magnetization M18 in the free layer 18 can be reversed by a simultaneous effect of the orthogonal magnetic fields HW1 and HW2.
In a “read” operation a selection transistor TR is opened by a bias voltage applied to its gate through a word line WL. The sense current of a relatively small magnitude flows through a bit line (BL), MTJ and TR. The current enables the resistance of the MTJ to be measured. By its comparison with the resistance of a reference memory cell (not shown), a magnetic state of the selected memory cell 10 can be determined. The memory cell 10 has a high thermal stability, good scalability and does not require special shape of the MTJ. However, the p-MRAM according to the prior art has an increased cell size, requires relatively high write currents IW1 and IW2, and suffers from half-selected cell problem.
FIG. 2 shows a structure of magnetic memory element 20 according to a prior art disclosed in the U.S. Pat. No. 7,432,574 (Nakamura et al.). The MTJ includes a pinned layer 14 and a free layer 18 both having perpendicular direction of magnetizations M14 and M18, respectively, separated from each other by a thin tunnel barrier layer 16. An insertion layer 24 is arranged between the tunnel barrier layer 16 and the free layer 18. The insertion layer 24 is made of a magnetic material with a saturated magnetization MS≦600 emu/cm3 that is necessary to obtain a GMR ratio of 20% or higher. Switching of the magnetization direction M18 in the free layer 18 is provided by a spin-polarized current IS running through the MTJ.
The spin-polarized current IS of a controlled polarity, magnitude, and pulse duration can reverse the direction of magnetization M18 in the free layer 18 by a spin momentum transfer. The spin momentum transfer is a phenomenon that occurs in current perpendicular to the plane (CPP) GMR devices that have cross-sectional areas of the order 104 nm2 or less. Strength of a spin torque is directly proportional to the IS current density running through the MTJ. The spin induced switching mechanism provides excellent cell selectivity in MRAM array; substantially lower switching spin-polarized current than that of the field induced switching MRAM and a possibility of substantial reduction of cell size.
However, maintaining a high precision of the current IS magnitude and pulse duration is extremely difficult. Moreover the spin-polarized current density required for switching is too high for integration with current CMOS technology. In addition, the high switching current running across the tunneling barrier layer 16 can create long-term reliability problems, for instance, its break down.
FIG. 3 shows a schematic cross-sectional view of MTJ element 30 with in-plane magnetizations M14 and M18 in the pinned 14 and free 18 layers, respectively, separated by a tunnel barrier layer 16. The MTJ element 30 employs a hybrid write mechanism according to a prior art disclosed in the U.S. Pat. No. 7,006,375 (Covington). An antiferromagnetic layer 32 controls a magnetization direction in the pinned layer 14 by exchange coupling between the layers. The hybrid write mechanism comprises producing a bias magnetic field HB along a magnetic hard axis of a free layer 18, and passing a spin-polarized current IS through the MTJ element 30 to reverse the magnetization direction M18 in the free layer 18 by spin-induced switching. The bias magnetic field HB induced by a bias current IB of a relatively small magnitude facilitates and accelerates the magnetization direction M18 reversal by the spin-polarized current IS. However the MTJ element 30 suffers from the same problems as other MRAM using in-plane magnetization direction in the free layer 18, such as poor thermal stability and scalability, relatively high switching current IS, necessity to use MTJ element of a special shape, and others.
What is needed is a simple design of MRAM having high thermal stability, excellent scalability, and low switching current that does not require a special shape of the MTJ element.